The present invention relates to an improved gate design for injection molding of rubber compounds to form rubber articles. More particularly, the present invention relates to an improved gate design for injection molding of fiber loaded rubber compounds with increased gate heating efficiency, reduced cycle time during the rubber part being injection molded, and improved fiber orientation in the rubber article.
In a typical rubber injection molding process, see FIG. 9, the uncured viscous rubber compound is introduced into the elongated barrel 12 of an injection molding machine 10 at ambient temperatures. It is advanced through the barrel 12 towards a mold 20 connected to the downstream end of the barrel 12, usually by either a rotating screw conveyor or a reciprocating ram or piston 14 disposed in the barrel 12. As the rubber compound advances, it is heated by heat conduction and mechanical shear heating in the barrel 12 to reduce its viscosity and render the rubber more flowable and amenable to subsequent injection into the mold 20. Typically, the less viscous the rubber compound, the more easily it flows through the conventional gate system 16 and more easily it fills the mold cavity 18 to produce a satisfactorily molded object.
Composite articles formed of an elastomeric base and reinforcing fibers are known in the art. The reinforcing fibers impart improved mechanical properties, such as abrasion resistance, tensile strength, compression resistance, and the like to the article. The fiber distribution and orientation are important factors which affect such properties. Controlling fiber orientation, therefore, is an important consideration to provide a reinforced article having the desired mechanical properties.
One known composite article is a tire tread. It is known that short-fiber reinforced treads with fibers oriented perpendicular to the tread surface can provide improved wear resistance and have excellent cornering resistance. Unfortunately, treads prepared by a conventional extrusion process have fibers oriented in the extrusion direction, that is, the tire circumferential direction. This orientation direction actually worsens the wear property because fibers oriented parallel to the rotational direction easily come off the tread surface.
An expanding die technology was developed to alter the fiber orientation direction. This technology is used to prepare short fiber reinforced tread extrusions with fibers oriented perpendicular to the tread surface. This technology is disclosed in WO 98/13185. WO 98/13185 is hereby fully incorporated by reference.
There are two steps involved in the expanding die technology. First a flat gate is used to orient fibers in the extrusion direction (or parallel to the tread surface). Then, the orientation direction of the fibers is changed to normal direction (or perpendicular to the tread surface) due to the folding action of the expanding die. Results showed that this technology prevented the fibers from orienting in the extrusion direction due to the folding action of the expanding die. However, it has been determined that the fibers are not exclusively oriented perpendicular to the tread surface, but there is also a lateral orientation of the fibers in the width direction of the tread.
The main reason for the lateral orientation in the width direction is due to the flat gate design combined with the expanding die. The pressure drop through the center path of the die is smaller than at the side path due to the additional pressure drop through the runner. This results in faster rubber flow at the center of the gate that creates a slight width direction extensional flow and lateral fiber orientation.
Another type of known gate is the lattice gate, disclosed in WO 98/56559. The lattice gate of WO 98/56559 minimizes differences in temperature and pressure that result in a parabolic rubber flow through the gate. This is achieved by a series of crossed flow channels. WO 98/56559 is fully incorporated herein by reference. However, for molding fiber-loaded compounds with a particular fiber orientation, the lattice gate fails to provide any particular orientation of the fiber. Due to the inclination angle of the flow channels and the flow of the rubber through the channels, the fibers are oriented at angles corresponding to the flow channels. The fiber-loaded ribbons coming out of the channels tangle with each other in a random structure that result in a random orientation of the fiber. Thus, the lattice gate alone cannot be used to prepare compounds with a specific fiber orientation.
The present invention provides an improved method and apparatus for injection molding rubber and, preferably, orienting fibers in a composite article, which overcomes the limitations of the known gate systems.
The present invention is an improved method of injection molding rubber. The method includes injecting a rubber through an injection gate to produce desired flow patterns in the rubber. The rubber is first directed to flow at cross angles through the injection gate, relative to the direction of flow of the rubber through the injection gate. The rubber is then directed to flow through the injection gate at a direction parallel to the direction of flow of the rubber through the entire injection gate.
In one aspect of the disclosed invention, the rubber is a fiber-load rubber. When the fiber-load rubber travels through the injection gate in accordance with the invention, the fibers are first oriented at the cross flow angles and then re-oriented to the direction of flow parallel to the flow direction of the rubber through the entire injection gate.
In another aspect of the invention, the distance of the cross-directional rubber flow, relative to a centerline of the gate, relative to the parallel-direction rubber flow, relative to the centerline of the gate, is within the ratio of 2:1 to 1:2.
Another aspect of the invention includes the step of directing the rubber through an injection gate exit into a mold cavity to form a series of folding planes perpendicular to the direction of flow through the injection gate exit.